By Ray Hainer

SATURDAY, October 9 ( — Women who stick to the Jenny Craig weight-loss program lose between three and four times as much weight as women who diet on their own, according to a new study published today in the Journal of the American Medical Association.

The study was funded by the Jenny Craig company, which provides counseling services and prepackaged low-fat foods to dieters through a nationwide chain of retail centers, or via phone and mail.

Women who ate the prepared foods and checked in weekly with a trained Jenny Craig weight-loss counselor in person or by phone lost about 20 pounds after one year, on average. A control group of women who ate a generic low-calorie diet and received less frequent advice from a nutritionist lost just 5 pounds.

After two years, the women who followed the Jenny Craig program were still about 15 pounds under their initial weight, which averaged about 200 pounds. (Women who received in-person rather than phone counseling lost slightly more weight.) In the control group, the average weight loss at the two-year mark was 4 pounds.

"If someone was motivated, and they adhered [to the program], this is not an unreasonable amount of weight loss to see over this period of time," says the lead author of the study, Cheryl Rock, PhD, a professor of family and preventive medicine at the University of California, San Diego. (Although Rock has no current financial relationship with Jenny Craig, she served on the company’s advisory board from 2003 to 2004 and led a smaller company-funded study that was published in 2007.)

But as Rock and her colleagues acknowledge, the study may warrant the same disclaimer found on the miraculous before-and-after photos that grace Jenny Craig ads: "Results not typical."

The key to any diet program is sticking to it. And many people who try programs such as Jenny Craig and Weight Watchers ultimately jump ship because they can no longer afford it, lose their motivation, or aren't getting the results they want.

The design of Rock's study likely defused those common reasons for quitting. The program and all of the prepackaged food were free—the food typically costs about $100 a week, in addition to a $359 enrollment fee—and the study volunteers were almost certainly more motivated than the average dieter.

"People don't sign up for a clinical trial unless they're fairly serious about losing weight," Rock says. Unlike regular Jenny Craig customers, the study participants went through an intake interview at a university medical center, had blood drawn, and agreed to participate in what they knew would be a two-year study.

Moreover, the study included just 442 women in all—a relatively small group from which to draw any definitive conclusions.

Can Jenny Craig dieters in the real world expect to drop 20 pounds in a year? "Most likely, the answer is no," according to Rena Wing, PhD, a professor of psychiatry and human behavior at Brown University, in Providence. Rock's results "probably represent a best-case scenario," Wing writes in an editorial accompanying the study.

Still, Wing suggests the findings shouldn't be discounted entirely. Meal-based weight-loss programs are significantly cheaper than surgical obesity treatments that are often covered by health insurance. If provided for free, structured programs like Jenny Craig may be a cost-effective way of encouraging weight loss and fighting obesity, Wing writes.